Poly(arylene sulfides) (hereinafter abbreviated as “PASs”) represented by poly(phenylene sulfide) (hereinafter abbreviated as “PPS”) are engineering plastics excellent in heat resistance, chemical resistance, flame retardancy, mechanical properties, electrical properties, dimensional stability and the like. The PASs are commonly used in a wide variety of fields such as electrical and electronic equipments and automotive equipments because they can be formed or molded into various kinds of molded or formed products, films, sheet, fibers, etc. by general melt processing techniques such as extrusion molding, injection molding and compression molding.
As a typical production process of a PAS, is known a process, in which an alkali metal sulfide that is a sulfur source is reacted with a dihalo-aromatic compound in an organic amide solvent such as N-methyl-2-pyrrolidone. As the sulfur source, a combination of an alkali metal hydrosulfide and an alkali metal hydroxide is also used.
In order to stably provide a good-quality PAS, it is necessary to strictly control polymerization conditions such as a molar ratio of an alkali metal sulfide to a dihalo-aromatic compound, a water content, a polymerization temperature and polymerization time on the premise that the purities of raw materials, secondary raw materials and the like are high and constant. For example, if a water content in the polymerization reaction system is too low, unpreferable reactions such as decomposition of a PAS formed tend to occur. If the water content is too high to the contrary, a polymerization rate is markedly slowed, or unpreferable side reactions are caused.
As the alkali metal sulfide of a raw material, is generally used its hydrate containing a great amount of water of crystallization. Further, these raw materials may be added to the reaction system as aqueous solutions in some cases. Accordingly, a great amount of water exists in the system at the point of time these raw materials have been charged into a reaction vessel. Upon the production of a PAS, thus, a dehydration step of heating and dehydrating a mixture containing an organic amide solvent and a sulfur source to control a water content in the polymerization reaction system is generally arranged as a step prior to a polymerization step.
The dehydration step is generally operated in the presence of an organic amide solvent that is a solvent for polymerization reaction and conducted until water is discharged out of the system by distillation, and the water content is lowered to generally 0.3 to 5 mol, preferably 0.5 to 2.0 mol per mol of the alkali metal sulfide. When the water content has become too low in the dehydration step, water is added prior to the polymerization reaction to regulate the water content within a desired range. After the water content is regulated, a dihalo-aromatic compound is charged into the reaction system, and the reaction system is heated, thereby conducting a polycondensation reaction.
In the dehydration step, the alkali metal sulfide reacts with water in the organic amide solvent, and hydrogen sulfide (H2S) is equilibriously dissociated and volatilized out. When water is distilled off by heating in the dehydration step, the water is discharged together with the organic amide solvent outside the system, or the organic amide solvent and water are separated from each other by distillation, and only water is discharged. At the same time, hydrogen sulfide formed is also volatilized out and discharged outside the system. The volatilization of hydrogen sulfide in the dehydration step causes the following problems in an industrial production process of a PAS.
First, since the amount of a sulfur source such as an alkali metal sulfide is varied by the volatilization of hydrogen sulfide, melt viscosities of product polymers vary every lot. In general, the quality of polymers formed of every lot varies according to, for example, a change of raw materials (particularly, an alkali metal sulfide and/or an alkali metal hydrosulfide), variations in the composition of raw materials attending on changes in the grade of PAS, or variations in the amount of hydrogen sulfide volatilized out attending on changes in the rate of dehydration under heat or changes of reflux ratio in a distillation column. In addition, even when the same raw materials are used, and production is performed under substantially the same conditions, melt viscosities of polymers formed vary between lots because the amount of hydrogen sulfide volatilized out in the dehydration step varies.
Second, it is difficult to stably produce a PAS having a high polymerization degree due to volatilization of hydrogen sulfide. For example, since a way of polymerization reaction between an alkali metal sulfide and a dihalo-aromatic compound is a polycondensation reaction between 2 components, it is generally desirable to regulate a molar ratio between both components to about 1:1 with high accuracy in order to provide a PAS having a high polymerization degree. Thus, the amount of hydrogen sulfide volatilized out in the dehydration step is predicted to control the amount of a sulfur source (alkali metal sulfide and/or alkali metal hydrosulfide) charged. However, it is difficult to control an accurate molar ratio between both components upon the reaction because the range of variations in the amount of hydrogen sulfide volatilized out is wide.
If the amount of hydrogen sulfide actually volatilized out is less than the predicted amount, a molar ratio of the alkali metal sulfide to the dihalo-aromatic compound becomes excessive, and so undesirable side reactions such as rapid decomposition reaction tend to occur. In order to stably produce a PAS having a high polymerization degree, thus, it is essential to strictly control and measure the amount of hydrogen sulfide volatilized out. However, it has been difficult to achieve the intended melt viscosity and thin a scatter of melt viscosity because the amount of hydrogen sulfide volatilized out in the dehydration step varies.
Some proposals have heretofore been made for the purpose of solving the problems attending on the volatilization of hydrogen sulfide in the dehydration step. For example, there has been proposed a process comprising determining the amount of hydrogen sulfide volatilized out in a dehydration step to find an amount of a sulfur source existing the reaction system with high accuracy (for example Japanese Patent Publication No. 33775/1988). According to this process, a molar ratio of the alkali metal sulfide to the dihalo-aromatic compound in the polymerization step may be fitted with high accuracy. However, it is necessary to introduce a special dedicated device for the determination of hydrogen sulfide volatilized out in the dehydration step, and the quantitative analysis brings loss of time.
There has been proposed a process causing hydrogen sulfide volatilized out in a dehydration step to be absorbed in an aqueous solution of an alkali metal hydroxide to recycle and reuse it in a dehydration step and/or a polymerization step of the next batch (for example, Japanese Patent Application Laid-Open No. 160833/1990). When the aqueous solution of hydrogen sulfide recovered is recycled through the dehydration step of the next batch, however, this process brings high energy loss because the amount of water to be dehydrated increases. When the aqueous solution of hydrogen sulfide recovered is recycled through the polymerization step of the next batch, the process involves the problems attending on a polymerization reaction in the above-described system the water content in which is high.
There has been proposed a process comprising causing hydrogen sulfide generated in a dehydration step to be absorbed in an aqueous solution of sodium hydroxide and conducting neutralization titration with 1N hydrochloric acid to determine the amount of hydrogen sulfide volatilized out (for example, Japanese Patent Application Laid-Open No. 160834/1990). However, the process comprising collecting hydrogen sulfide in the aqueous solution of sodium hydroxide to determine it requires to introduce a dedicated device for the determination, and moreover an operation for determining is newly developed to reduce production efficiency. In addition, when the aqueous solution of sodium hydroxide containing hydrogen sulfide recovered is recycled through the reaction system, problems attending on increase in sodium hydroxide and water arise from the viewpoints of polymerization reaction and the quality of a PAS to be formed.
There has been proposed a process comprising recovering hydrogen sulfide volatilized out during a dehydration step by causing it to be absorbed in an organic amide solvent outside the system, in which the dehydration step is being conducted, and reusing the hydrogen sulfide recovered as a sulfur source in a polymerization reaction (for example, Japanese Patent Application Laid-Open No. 286861/1997). According to this process, the hydrogen sulfide volatilized out in the dehydration step can be recovered and reused, thereby solving various problems attending on the volatilization of hydrogen sulfide and producing a PAS little in variation of melt viscosity and stable in quality. However, even in this process, the amount of the volatilized hydrogen sulfide absorbed in the organic amide solvent varies, and there is a demand for developing a method for more accurately grasping the amount of hydrogen sulfide.
Among the qualities of a PAS, a melt viscosity is one of the most important qualities. It is known that a charge ratio (hereinafter abbreviated as “P/S ratio”) of a dihalo-aromatic compound to an alkali metal sulfide strongly affects the melt viscosity of the PAS. However, in a production process of a PAS, comprising adding an alkali metal hydroxide as needed, and heating and polymerizing a dihalo-aromatic compound and an alkali metal sulfide and/or an alkali metal hydrosulfide in an organic amide solvent, there has not been yet proposed an industrially adoptable and excellent method for fixedly controlling the P/S ratio.